Implantable defibrillators are known in the prior art in which the spectrum of heart rates is divided into several distinct bands. Such bands may include normal sinus rhythm, tachycardia and fibrillation.
We have found that it would be extremely useful to divide the spectrum of heart rates into five bands, including bradycardia, normal sinus rhythum, slow tachycardia, fast tachycardia and fibrillation. However, when rhythms occur that have a combination of intervals, i.e., where the rhythm is oscillating around a border between two tachyarrhythmias, the implanted device must decide which arrhythmia is present. Further, the device must detect sinus rhythm when it occurs.
The present invention concerns a combined antitachycardia pacemaker/defibrillator which is implanted to treat potentially lethal arrhythmias. However, antitachycardia pacing can be an unpredictably prolonged therapy, and can result in leaving the patient in jeopardy if more definitive therapy is not used within a short period of time. One solution to this problem would be to severely limit the programability of the antitachycardia pacing feature. However, this is not a desirable solution.
Another potential problem in an automatic tiered therapy defibrillator, is that if a rapid arrhythmia such as a high rate tachycardia or fibrillation is detected, and the associated therapy is delivered, the device must know when to stop delivering the therapy. The device could be programmed to stop delivering therapy when the original detection criteria are no longer met.
Currently available defibrillators have a single rate cutoff. If it is exceeded, therapy is delivered. Normally if the average heart rate is high enough to be considered a tachycardia, an automatic device will provide tachycardia therapy. However, it is possible to have rhythms that have an average tachycardia rate but have an alternating pattern of intervals and should not be treated. These rhythms are sometimes called bigeminal rhythms. An example of a bigeminal rhythm is 600 msec/300 msec/600 msec/300 msec/etc. The average interval is 450 msec. If the tachycardia detection criterion is 500 msec, the device would inappropriately diagnose tachycardia.
Some prior art antitachycardia devices require a sequential number of intervals below the interval criterion for tachycardia. However, this makes it difficult to detect arrhythmias unless the arrhythmia rate is very stable.
It is, therefore, an object of the present invention to provide a method for determining arrhythmia hierarchy in an implanted defibrillator which has a number of distinct bands.
Another object of the present invention provide definitive therapy for patients suffering from tachycardia, using an implanted combined antitachycardia pacemaker/defibrillator.
A further object of the present invention is to utilize an arrhythmia detection method using hysteresis, in which there is one rate cutoff for deciding to deliver therapy and a separate, lower rate cutoff, for deciding to cease giving therapy.
A still further object of the present invention is to provide arrhythmia detection inhibition with low tachycardia rate averages, but interval alternans. In this manner, the system will keep track of the ratio of sinus intervals to tachycardia intervals, and will require more tachycardia intervals than sinus intervals in order for an arrhythmia to be detected. Therefore the presence of a tachycardia can be quickly determined, without inappropriately detecting a bigeminal rhythm as tachycardia.
Other objects and advantages of the present invention will become apparent as the description proceeds.